IN THE TWO MAJOR TECHNICAL PUBLICATIONS ON THE ION-PROBE TECHNIQUE IT WAS RECOGNIZED THAT THE MEASURED DATA FAILED TO CONFORM TO THE THEORETICAL MODEL YIELDING BETWEEN 50-75% APPROXIMATIONS (KUHTREIBER AND JAFFE, 1990 J. CELL BIOL. 110: 1565-1573. AND SMITH, SANGER AND JAFFE, 1994 METHODS IN CELL BIOLOGY 40: 115-134.) WE HAVE NOW DEFINED THE SOURCE OF THE ERROR. IT CONTAINS TWO MAJOR COMPONENTS. 1. THE ORIGINAL EQUATIONS ASSUMED THE ZERO POSITION WAS PLACED AT AN INFINITE SOURCE. THIS WAS CLEARLY INCORRECT BEING DEALT WITH BY REJECTING THE INITIAL CALIBRATION CURVE DATA. AN APPROXIMATION TO LINEARITY APPEARS AS THE DISTANCE DECREASES AND THE INITIAL ERROR BECOMES LESS SIGNIFICANT. MODELING THE CURVES AND INCORPORATING DISTANCE CORRECTION FACTORS REDUCES THIS ERROR. 2. MORE IMPORTANTLY TO THE PRACTICAL APPLICATION OF THE PROBES IS THE ISOLATION OF THE MAJOR ERROR TO THE TIME CONSTANT OF THE IONOPHORE. ALTHOUGH THE ABOVE POINTS MAKE LITTLE DIFFERENCE TO THE USE OF THE PROBES IN BIOMEDICAL RESEARCH THEY DO ALLOW US TO PLACE A CORRECTION FACTOR ON THE RECORDED DATA. MOST IMPORTANTLY THEY DEMONSTRATE THAT WE CONSISTENTLY TEND TO UNDERESTIMATE THE FLUX MAGNITUDE.